Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and a rotor. The rotor is coupled to the nacelle and includes a rotatable hub having one or more rotor blades. The rotor blades are connected to the hub by a blade root. The rotor blades capture kinetic energy from wind using known airfoil principles and convert the kinetic energy into mechanical energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
The particular size of the rotor blades is a significant factor contributing to the overall capacity of the wind turbine. Specifically, increases in the length or span of a rotor blade may generally lead to an overall increase in the energy production of a wind turbine. Accordingly, efforts to increase the size of rotor blades aid in the continuing growth of wind turbine technology and the adoption of wind energy as an alternative and commercially competitive energy source. Such increases in rotor blade size, however, may impose increased loads on various wind turbine components. For example, larger rotor blades may experience increased stresses at the connection between the blade root and the hub, leading to challenging design constraints, both characterized by extreme events and fatigue life requirements.
The likelihood of structural failure due to fatigue at the rotor blade joint is typically increased by the presence of high stress concentration between the load bearing components, manufacturing defects, unexpected loading events or deterioration of the joint. Loss of preload can also occur in the bolted joint which is known to reduce fatigue life. To endure the load envelope specific to the rotor blade root, various methods and systems have been devised and implemented to improve the connection between the dissimilar materials intrinsic to the rotor blade components. For example, some systems consist of a blade root having a flange that is bolted to the hub. More specifically, the root flange may be bolted to the hub via a plurality of T-bolt connections. In such an embodiment, a plurality of root bolts are secured to the blade root by inserting the bolts into a plurality of corresponding barrel nuts configured perpendicularly in the blade root. Thus, the strength of the bolted connection may be increased as the number T-bolt connections increases. In further systems, steel inserts or rods may be bolted directly into the composite blade root to provide increased stiffness to the bolted connection. In still additional systems, low-cost, low-density foam may also be inserted between the bolts and inserts.
Not all such configurations, however, maximize load transfer between the composite and the metal materials, which may cause a variety of design problems such as high concentration factors, structural discontinuity, and ovalization issues due to the Brazier effect. The Brazier effect occurs when the blade root is subjected to bending and the longitudinal tension and compression (which resists the applied load) tends to flatten or ovalize the cross-section of the root. In addition, the number of root bolts that may be used to join the blade root to the hub is inherently limited to the number of barrel nuts that can be circumferentially spaced about the blade root.
Thus, there is a need for an improved rotor blade assembly suitable for securing the rotor blade to the hub of the wind turbine that provides enhanced root stiffness, thereby enabling further scaling of the wind turbine rotor blades.